The present invention relates to a pulsed amplifying system and method; and more particularly, to an improved pulsed amplifier and method of the type using energy storage capacitance for supplying pulsed power supply current to a load.
Amplifiers which operate in modes class B or C have a DC bias which leaves the device non-conducting in the absence of an AC drive signal. When a drive signal is impressed on the input electrode (usually a base, emitter, or gate) the amplifier conducts over approximately half of the input AC cycle. The DC component of this current is drawn from the power supply, which is the ultimate source of energy for the AC output signal. For pulsed RF amplifiers, the DC drawn by the transistor is in the form of current pulses, having a duration, rise, and fall times of the RF envelope. It is generally required to have an energy storage capacitor which supplies this pulsed DC current; and the power supply then provides only a relatively smooth recharge current to the capacitor.
For systems with long pulse widths, such as in the neighborhood of 100 microseconds and longer, for example, the energy storage capacitor can become quite large, and consume a significant portion of the total system weight because it must supply all the energy to the transistor amplifier for a single pulse, and do it without much voltage droop. The initial voltage on the energy storage capacitor is chosen to be the maximum that allows reliable operation of the transistor. The capacitor is sized to hold voltage droop to the level which still provides acceptable power out of the amplifier. To a first order approximation, the transistor output power droop is inversely proportional to the square of the capacitor voltage droop. Thus, if the capacitor is large enough to hold voltage droop to five percent, for example, the amplifier output power droops approximately 10 percent, and 90% of the stored energy remains in the capacitor, unused. It is obviously inefficient to store 90% more power in a capacitor bank than is used by the amplifier. This inefficiency shows up in system weight, size, and cost.
In vacuum tube radar transmitters, the energy storage problem is sometimes alleviated by the use of a "line-type" modulator. The line-type modulator uses a pulse forming network (PFN) instead of a brute force capacitor bank; and the capacitors in the PFN are completely discharged on each pulse. This technique, however, requires a high current electronic switch for the PFN, such as a thyratron, an SCR, or a thyristor, for example. For a solid state transmitter, the line-type modulator represents not only additional system complexity, control circuitry, cost, power consumption, but also constitutes a single thread failure mode affecting all the transistors it would modulate.